Ecology: Lecture 17 Community structure

Vertical stratification in lakes

River Continuum concept (pictures from Stroud Water Research Center) Headwater Mid-sized stream Large river

Biological Structure Species dominance Numbers Impact Dominant groups in stream sample? Impact Impact of introduced lampreys on fish Keystone predators such as Pisaster ochraceus (type of sea star)

Biological Structure Species diversity Number of taxa (families or species for ex.) First approximation Shannon diversity index considers 2 factors: Number of taxa Evenness (or “relative abundance”) For a given number of families, the Shannon index is higher when there are approximately equal numbers of individuals in each group (more even distribution of individuals)

Rank abundance of species Method of examining evenness Pi values are on the y-axis Groups placed in order of pi values

Effects of population interactions on community structure Competition and community structure Focus: Zebra mussels Invasive Freshwater filter-feeder Attach by byssal threads Encrust any submerged hard surface Envirozine (Canada)

Zebra mussels: major effects Outcompetes native clams for space (how?)  zooplankton (efficient filter-feeding)   other filter-feeders or fish that feed on zooplankton  water clarity   growth of benthic algae Why so successful? Good “fit” with new environment No native parasites or predators www.boatnerd.com

Local example: Himalayan blackberry Note robust stem with heavy thorns Grows tall and dense Outcompetes native shrubs Blocks light for shade-intolerant tree seedlings Net result:  in community diversity of both plants and animals (Why animals too?) Also limits animal movement

Two hypotheses for level of community diversity Bottom-up hypothesis Top-down hypothesis

Bottom-up hypothesis Defined: The abundance and diversity of primary producers (the “bottom”) affects the diversity at higher levels of the food web. Stream-related hypothesis: Diversity in different parts of a river is a consequence of the diversity and abundance of primary producers. Diversity: High diversity in the mid-reaches from multiple sources  more feeding niches. Review: Sources of primary production along river continuum Abundance: Higher total primary production  increase in food resources  reduction of competition among taxa (i.e. reduces the chance of competitive exclusion)

Top down hypothesis Defined: Levels of diversity are controlled by predators Classic example: Pisaster ochraceus, the ochre star (native inhabitant), is a keystone predator in the community (Bob Payne)

Keystone predator: Pisaster ochraceus Pisaster maintains (promotes) diversity Mussels would otherwise dominate mid-intertidal

Pisaster ochraceus predation and diversity (2 mechanisms)

Keystone predator (invasive) Non-native rainbow trout Keystone predator decreases diversity in this case Understand the mechanism! Step-by-step…